Tamper-resisting protective device



Aug. 28, 1951 E. H. TAYLOR 2,565,827

TAMPER-RESISTING PROTECTIVE DEVICE Filed sept. 18, 194e /l zg il. vw* 24 IH 12v/41g y IN VLN TOR. %Z7fw EA@ I Patented Aug. 28, 1951 TAMPER-RESISTING PROTECTIVE DEVISE Elmer H. Taylor, Newburyport, Mass., assig'nor to The Chase-Shawmut Company, Newbury'- port, Mass., a corporation of Massachusetts ,Application September 18, 1946, Serial No. 697,662

8 Claims. (C1. 20G-119;)

This invention relates to improvements .in thermal protective cut-outs 4vfor electric circuits. More particularly it relates to such cut-outs of the fuse plug ty-.pe wherein the fuse `plug is mounted in a cut-out .base receptacle which .be equipped with means :for -coaction with the plug to make it diicult to restore a circuit through the assembly, follow-ing lan interruption by the fuse plug, excepting by substitution of la new `fuse plug .having a rated current-.carrying capacity .not greater than what has been predetermined :as proper for any particular installation.

The lprevalent and dangerous practices -of .ruseltampering and over-fusing are the direct and contributing causes -of numerous fires each year, which -take human lives :and `destroy and. damage inestimable amounts [of property. 'The -sole purpose 4of a `fuse ina-n :electric acircuitis to ensure against the lcontinuance of a dangerous circuit abnormality for fa ,period .long enough to destroy wiring insulation orr .to fcauserrdama'ge .to apparatus which may vbe connected in the rcircuit. When .a fuse -of predetermined proper rating is in-a circuit, it stands las .a sentinel ever ready yto interrupt the lcircuit in response to any circuit abnormality whose continuance `beyond a gpredetermined period of time would vcreate a re hazard or be otherwise dangerous and-objectionable. But, when, through ignorance 'or design, z

a .circuit is rover-.fused 'by :insertion .in :the -base :receptacle fof a fuse having -a current-carrying rating substantially above what has :been prede- .terrnined :as 4the maximum safe operating cur- `rent Pfor any particular circuit, the :substituted -fuse cannot :protect 'the circuit because the fuse is adapted to carry currents substantially above what has 'been :predetermined as lthe :maximum safe current for lthe particular circuit.

Similarly, following blowing :of a (fuse, .if Vthe ,interruptedcircuit is completed through the cut out assembly .by make-shift .means inserted -in .the lassembly' to ybridge ,or otherwise .by-pass the blown .-fusible elementof the efuse plug, 4that pro- -tection which it is the sole purpose .of the ,fuse

in the `circuit Ato provide is lostif, `as is usual, ythe make-shift means has rcurrent-'carrying .capacity appreciably greater than that of the fuse tplug ,prior .to `the blowing thereof.

The serious nature of .the problems of yover-- :fusing :and ,fuse-tampering :are recognized (and emphasized ,-in ,the VNational Electrical Code,

shortly .-to .become effective, .which includes =man datory provisions 1making itreguisitein all new `.fuse .plug installations,l to -employ ronly so-called type S- fuse plugs which have lbeen approved or tentatively 4approved by the yElectrical Committee Vas adapted, when mounted in a cut-outbase receptacle, to make fitacceptably difficult both t0 over-:fuse a circuit and to bridge or otherwise 'by-Y pass .the fusible element of a .blown fuse in a cutout y assembly.

Numerous fuse plugs .and ,fuse plug assemblies as proposed lheretofore embody means for coact- .ing in ,a cut-out assembly vto ,make it difficult `either .to over-fuseacircut or to 'bridge or other# wise by-pass the fusible .element of a'blown fuse plug, :by .make-.shift means inserted 'in the cutout assembly. However, it has been recognized ,that lany satisfactory or generally lacceptable ,program to .minimize the hazards and the dire eiiects of iluso-tampering and over-'fusing must take into consideration the some two hundred million standard .Edison 'base fuse plug recep- Jtacles embodied fin ,present installations. The ,mentioned :mandatory lprouisions of the National Electrical VCode apply vonly 'to new installations, yet .ithas been generally recognized by those `responsible :lor the Code provisions and by ,fuse manufacturers .and ,large users .of jiuses, such as public utility organizations', that any fuse plug .acceptable .as satisfying the requirements of the @Code `for ynew 'installations should 'be adaptable also or operative .embodiment in any of the presently installed standard Edison base cut-out re# ceptacles.

.The .so-called type S ,use plug whichpresently is designated .by the Code, AVunder its -sa'id manda'- tory provisions, "includes an adapter which is ,a necessary lpart of -the use Yassembly and which makes Tit Ipossibile to .mount 'the -type S 'fuse plug insany standard'gbase receptacle. But the type S plug, with 'its adapter, costs more than 'the conventional ruse, and public utility organizations, Whose v responsibility 'it iis to replace blown 'fuses in a large ,percentage of the standard :base re- -`ceptacles nowin use, realize 'that many of Jtheir customers may :prefer the lower .cost conventional 'fuse replacement. Hence, an objection `of A,tlfie'public `utility ,organizations to Lthe type :S fuse tplug 'is that 'both conventional and 'type 'S `fuses, the latter with their adapters, would have .to be .carried n stock. Also Lthere has =been general.criticism oi the type S fuse Aplugbecause certain 4.peculiarities o'f 'its structure involve special .manufacturingprocedures which .would tend to restrictmanufacture of special parts of therplug and adapter.

VVAWhile 'it'is guiteuniversally agreed Athat ,it 'is liiglily .desirableand essential that it be made mandatory in new installations to employ fuse plugs which effectively resist fuse-tampering and over-fusing, very strong opposition has been raised to designation, under the mandatory requirements, of a fuse plug structure which cannot be open to general competition in its manufacture and which requires a relatively expensive adapter to permit operative mounting of it in a standard Edison base receptacle. Even the International Association of Electrical Inspectors, which has been a primary force back of the movement to obtain acceptance,.under the Code, of the type S fuse plug and adapter structure, has felt called upon to advocate an intensive educational program which it airmatively states must be put into effect if the type S fuse is to be successful.

Another objection to the so-called type S fuse plug and adapter assembly is that the fuse plug, when mounted in a base receptacle, projects from the receptacle substantially beyond the plane of maximum projection of the conventional fuse plug. Actually the type S fuse projects approximately one-quarter of an inch further out from a ybase receptacle than does the conventional plug. Inasmuch as many of the present installations have the base receptacles and fuses enclosed within containers whose covers, when closed, just slightly clear the outer ends of the conventional fuse plugs, the substitution of type S fuse plugs and adapters makes it impossible to close the covers of the containers.

It is among the objects of the invention to provide a fuse plug which may be mounted in the standard Edison-base receptacle either with or without means for coactionV with the fuse plug to make the assembly acceptably resistant to fuse-tampering and over-fusing. I provide a fuse plug having a standard screw-shell contact for screwing directly into the screw-shell contact of the standard cut-out base receptacle, and having a standard center Contact for directly engaging the center contact of the base receptacle, so that the fuse plug requires no adapter for the operative mounting of it in the standard cut-out base receptacle and which projects from the base receptacle no more than the conventional fuse plug. However, the body portion of the fuse plug, which carries its center contact, has reduced diameter for nicely fltting within a tamper-resisting sleeve member which may precede the fuse plug into the base receptacle and which has an annular portion for extending between the screw-shell contact and the reduced portion of the fuse plug for isolating from each other the center and screw-shell contacts of the receptacle, and for coaction with the fuse plug in making the assembly acceptably resistant to fusetarnpering and over-fusing.

Another object is to provide a fuse plug having an insulating body which may be economically molded or otherwise formed in one or more parts according to conventional procedures. and which may have an exteriorly smooth body portion for coaction with tamper-resisting means interiorly of a fuse plug receptacle to effectively resist fuse-tampering and over-fusing.

Still another object is to provide a fuse plug assembly comprising a fuse plug, a standard cutout base receptacle and a tamper-resisting insulating sleeve insert, wherein both the fuse plug and the sleeve are exteriorly threaded for screw- .ing directly into the screw-shell contact of the vbase receptacle, and wherein' the fuse plug has vaxial extent approximating the axial extent 'of the conventional fuse plug, and has a center contact-carrying body portion for carrying the center contact of the plug axially through the sleeve into direct engagement with the center' contact of the base receptacle, whereby the fuse plug projects from the base receptacle no more than does the conventionalfuse plug.

Yet another object is tov provide a tamperresisting fuse plug having a standard screwshell contact for screwing directly into the" screw-shell contact of a standard cut-out base' receptacle, and having an insulating body portion projecting axially within and beyond said screwshell contact of the fuse plug and constituting one Wall of a substantial annular chamber interiorly of the screw-shell contact of the fuse plug.

A further object is to provide a tamper-resisting fuse plug having means thereon for coaction with a tamper-resisting insulating sleeve which is mountable within a standard cut-out base receptacle, said fuse plug having standard screwshell and center contacts directly engageable respectively with the conventional screw-shell and center contacts of the base receptacle both when the tamper-resisting sleeve is in the base receptacle and when the sleeve is absent from the receptacle.

Another object is to provide a fuse plug cutout assembly comprising a fuse plug, a cut-out base receptacle and a tamper-resisting sleeve, wherein the tamper-resisting sleeve and cut-out base may be integrally molded of insulation material, or the tamper-resisting sleeve may be separately formed for embodiment in the cut-out base as an insert, said base receptacle in either case having a screw-shell contact for surrounding the tamper-resisting sleeve, in spaced relation thereto, and for directly receiving a screw-shell contact on the fuse plug, a body portion of the fuse plug having dimensions for tting within the tamper-resisting sleeve, and carrying a center contact for direct engagement with the center contact of the cut-out base receptacle.

An additional object is to provide a fuse plug cut-out assembly comprising a fuse plug, a cutout base receptacle and a tamper-resisting sleeve, wherein the tamper-resisting sleeve is within the base receptacle and has a substantial outwardly projecting portion which is exteriorly threaded and which has outside diameter only slightly less than the inside diameter of the screw-shell contact of the base receptacle, and wherein the fuse plug has a bare screw-shell contact for screwing into the receptacle with the screw-shell contact of the receptacle-engaging it exteriorly and with said threaded portion of the sleeve engaging it interiorly.

Another object is to provide a tamper-resisting generally tubular insulating member exteriorly threaded at two differing outside diameters thereof so that one of said threaded diameters is adapted to screw into the screw-shell contact of a fuse plug base receptacle and the other of said threaded diameters is adapted to screw into the screw-shell contact of a fuse plug when said screw-shell contact of a fuse plug is screwed 1int? said screw-shell contact of the base recepac e.

Itis, moreover, my purpose and object generally to improve the structure and adaptability of tamper-resisting fuse plugs and assemblies and more especially such fuse plugs and assemblies which may be designed also to resist over-fusing.

In the accompanying drawings:

Fig. 1 is a cross-sectionalv view of a fuse plug iiounteev in a Standard out-out baise" reoeptacie and' including inserted Ii'ieins"v Within the recep'' tacle coacting with' the' fus'eplug to' resist fusee" tampering ,Y I

Fig. 2 is a View', similart'o Fig. 1, but shovvir'ig;` the fuseD plug operatively mounted in the' base"l receptaclfe Without the' said inserted coacting ineans of Fig. l; f l

Fig. 3is" an end elevation of the assembly of Fig. l, in cross-section on line' 3-'-3;V 4 and 5 are" similar elevational views" of two fuse plugs embodying features of my v'ention and having center contact-carryingbojdy' parts Whoseexterior diametersv are appreciably different soI that they are' not interchangeable' in any particular tamper-resisting insulating?v sleeve element; U

Figs. 6 and '7 are similar' elevational viewsT ofA two tamper-resistingU insulating sleeve elements" having interior diameters `for nicely aji'zcorrimo'dating, each a particularone ofl the' fuse' plugs'lof Figs. 4 and 5, the sleeve element having the" snaller interior diameter' being adapted to rel j'ect the fuse piug having thelarger exterior' diameter;

Fig. 8 isa bottom plan View of my improved fuse plugsh'ovving serrations at the e'nd of its center contact-carrying body part;

Fig.Y 9 is an elevation of the inner end' por# tienofv the plug of Fig. 8; Y

Fig. 10 is an elevation showing how rr'yV tame" perlresisting sleeve element may be serrated at' its outer end, to increase'its breaking and cut-n ting-l e'ect, in coa'ction with a fuse plug; and

Fig.- 11 is a cross-sectional view through a cuteA out base receptacle having m'y tamper-resisting sleevformed as an integral part of the insulate' ing body' of the cut-out base'.I

Referring to the drawings; the cut-outl base infr' dic'atedgenerally at I0' may be of any suitable design and construction but preferably will be a? standard Edison base, as shown, having the screw-shell contact l2 electrically conne'ctedtd' the terminal element I4 to which onek line con; ductor may be connected by meansof the binding screws I6. The usual center contact of the bas'e" is indicated at I8 electrically connected to* te'rminal element 20 which isV adapted for connection zo'4 the other line conductor by the binding' screv My improved fuse plug, tamper-resisting typen-indicated generally/at 241, andit isa fea-i ture that the plug has a screw-shell contact- 256;' for screwing directly into the screw-shell coii' tact l2 of the base receptacle,r and has a cer-iter' contact' 28 for directly engaging theV center coni' tact 118 of the base receptacle. It should' be note'dl thatv7 the screw-shell contact- 26 and the ceri-tori contact 28 have the same relation to e'a'clf'iz othe'i as'A the' screw-shell and center contacts! of the conventional fuse plug, and that the'- a-ziali over?- allfext'ent of my improved plugmay be thes'arn as that of the conventional fuseV plug- Fig'sr' illust-reces me mounting of the fuse' plug in the' base receptacle withoutl any adapter or other' means additionally inserted' in the' rebept'acle'.l Hence, it will be readily seen that my improved fuse plug may be operatively mounted inf an'y of the two hundred minion presently installed standard Edison' base' receptacles substantially in the precise manner that regi'ilarv fuses" for years have been mounted, and` With ro greater aXial projection from the receptacle than the conventional fus'e plug. Also,- niy improved plg as mounted in Fig. 2, provides the same degree of procedures.

eirooii'*.prestation-inet ijiae been giifeii by aeoof veritional fuse rplug of the sain-erating'. Y

n order' tor'r'eet` tha-requirements of thev Na` tory for new fiise plug iosiaiiatioos, my improvedextends axially^ Within and beyoiid the screw# shell contact 26', and this body` part Sii' has-exterior diameter substantially lessthan the-wintenor' diameter or' thel screw-shell contact 2s,-

tlat an annular' c'harrilder'Y 3'2` of considerabla depth' iritervenesl between the body part 36 and. the` sorewshe'll con-tact 2`5fof the fuse plug.v The c'eiiter'coritact 286i the' fuseplugj is mounted ati previou'sl3'r pointed out; is substantially th'e;

betiveerij he screw-shell' and center contacts of? theordinary' conventional' fusepl'ug. l According" to the invention, the insulating body parte 3516i the fusephgis` adapted for coactionl pl cptacle, with my tamper-resisting insulating sleeve element 354i in position* Within the basev receptacle and projecting into the annular charnloer32'of the fuse pl'ugin closely' surrounding relation tothe insulating body: part 33 of the plug.y

ly'cornparisonof'Figs. 1'- ajnd 2, it vvill beseen that rfriyfim-proved fuse plugY seats in' the base recep` tacle in precisely' the samel manner both with and Without the tamper-resisting sleeve 3d in pl c; andits'faial projection from the receptacle isl the same' in* each case. Both with and Without the tamper-resisting sleeve 34, the center contact of the plug directly engages the cen' ter contacter the receptacle. However, with the el'eev'e place',- as-`in Fig. l, itsy substantial annu-` lai'eitent Within the screw-shell contact 26, and its closely surrounding relation` to the insulating" oodypa-'r't aefoi me plug mage it extremely eifricult"y to=bridge or otherwise by-pass the fusible element ofthe plug' either by make-shift means inserted in the base'preceptacle or by displaced portions of either of the inter-engaged screw# shell conta-cts'. Y y

Furthermore; both my improved fuse plug 24 aridj my tamperlresisting insulating sleeve ele--Y I'eit 3:4"m`ay be readily and economically pro duced according to conventional manufacturing The Vfuse plug body may be integrally formed in one .piece of any suitable insulating material' such as porcelain, glass, plastic, ofv a composition: material'. However, it is not necessarily'made inv one' piece' nor of one mate`v rial' throughout', but may, if desired, be partly of one material andl partly of `another material. As hereinv shown, the insulating body isV an in-VV tegal onepiec'e structure `having the head part S'includihg the annular'flange` 33; with thepre; viously-nientioned body part Svof smaller diarrr`A eterqprojecting' axially from thehead;

Tliebody is' interiorly chambered to provide a' relatively deep well MIl whichv is open through the"liead` en d of theI body and covered by the: customary disc of mica" 4'2, orV the like, held ii eioeiby nie'ririg eieriieni as' wi'iieii conveniently ay'be'cluped overl theilage 3'8 which r'ay'l' 'A mounted inthe standard cut-out base re" ascesa?- serrated at the region of crimping to lock the ring element against rotation on the plug body. The body part 30 may be variously shaped exteriorly but preferably has the generally cylindrical smooth surfaced exterior as shown for nicely fitting within the complementary axial hole through the insulating sleeve element 34. The center contact 28 of the plug extends through the bottom wall of well 40 at the Yend of body part 30, and a usual deposit of relatively high temperature melting solder 44 or the like electrically connects the center contact 28 to the fusible element 46 whose one end is embedded in the solder 44 and whose other end extends into one of a pair of recesses 48 interiorly of the head part 36 and on opposite sides of well 40, where this end of element 46 is suitably anchored by means of a deposit of relatively high melting temperature solder 50, or the like. The screw-shell contact 26 of the plug conveniently and eiectively may be mounted on the plug body by means of an annular inturned ilange 52, at one end of the screw-shell contact, which seats on the shoulder 54 at the juncture of the head part 36 and the body part 30 of the insulating plug body, the iiange 52 having oppositely arranged tongues 56 for extending each through an opening 58 into a different one of the interior recesses 48, where one becomes both anchored and electrically connected to the fusible element 46 by the said solder 56 and the other is anchored in the other recess 48 by a similar deposit of solder 60.

My tamper-resisting insulating element may be variously shaped so long as it provides a needed axial passage for projection of a plug center contact into direct engagement with a receptacle center contact. Figs. 1, 5, 7 and l0 show a preferred form of tamper-resisting element 34 which is generally tubular on toroidal in shape. It is adapted for securement as an insulating sleeve within the screw-shell contact I2 of the cut-out base with the inner end of the sleeve surrounding the center contact I8 of the cut-out base and with the main body of the sleeve projecting outwardly within the screw-shell contact I2.

It is a feature that the inner end portion of sleeve element 34 has outside diameter slightly greater than that of the outer end portion of the sleeve, and that both the larger and the smaller diameter portions are exteriorly threaded as at 62 and 64. The larger diameter portion 62 is adapted to screw into the Vscrew-shell contact I 2 of the cut-out base while the smaller diameter portion 64 is spaced inward of screw-shell contact I2 only enough to provide a narrow space for receiving the screw-shell contact 26 of the fuse plug 24, with the screw-shell contact I2 threadedly engaging the screw-shell contact 26 exteriorly and with the threads of the smaller diameter portion 64 of sleeve 34 in close proximity to the threaded walls of the screw-shell contact 26 interiorly. It will be obvious that there is practically no possibility of effecting a bridging of the center contact and the screw-shell contact of the base receptacle without the bridging means preventing screwing in of the fuse plug, or if the fuse plug is screwed into the receptacle, any bridging device necessarily will be severed and renderedinoperative as a bridge. Any suitable means at the periphery of the larger diameter threaded region 62 may be provided for locking the sleeve lagainst removal after it has been initially screwed to an operative position within the screw-shell contact i 2. As shown, this locking means isa piece of wire 66 anchored at oneend in a peripheral recess 68 and projecting resiliently outward slightly beyond the periphery, with the outer end sharpened and in rposition lagging behind the anchored end when the sleeve is being screwed into the screwshell contact I2. Hence, the free sharp end of wire 66 rides freely on the inner surface of the screw-shell contact I2 when the sleeve element 34 is being screwed into the receptacle but resists unscrewing by digging into the wall of' screw-shell contact I2 when attempt is made to turn the sleeve in direction to remove it from the receptacle. Or the wire 66 may engage in a cut-out 10 in the shell contact I2 when the sleeve is seated so that any attempt to unscrew the sleeve is effectively resisted by the wire 6s engaging a wall of the cut-out l0.

Each insulating sleeve element 34 has an axial hole 'l2 therethrough of size and shape to nicely accommodate the body part 36 of the fuse plug, while the main extent of the sleeve element enters the annular chamber 32 of the fuse plug as a barrier for isolating from each other the center and screw-shell contacts of the assembly.

It will be obvious from an examination of Fig.

l that insertion of a coin or a wad of tin-foil within the sleeve element 34 cannot complete a circuit between the center and shell contacts of the assembly, assuming that the fusible element 46 of the plug has been blown. On the other hand, if a wire or other conducting element is inserted into contact with both the center and shell contacts, the body part 36 of the plug, entering the sleeve element 34 with close clearance, and the screw-shell contact 26 of the plug entering between the sleeve element and the screwshell contact of the base, will break or shear any such element having a size capable of bridging the assembly. In other words, the coacton of my improved fuse plug and my tamper-resisting insulating sleeve element 34 in a standard Edison base fuse receptacle provides a degree of resistance to fuse-tampering meeting all of the Code requirements in this respect.

Over-fusing may be effectively resisted with my improved fuse plug by slight variation of the body parts 30 of plugs having diiferent rated capacities, and by a corresponding variation in the diameters of the axial holes in my tamperresisting sleeve elements 34. For example, the body part 3U of fuse plugs having 0-15 ampere rating may have exterior diameter of 46- of an inch, while the body part 3E! of fuse plugs of 1630 ampere rating may have exterior diameter 0f 5% of an inch. The axial holes in sleeve elements 34- similarly may be of two slightly different diameters so that the sleeve element 34 for a 0-15 ampere fuse plug will reject a fuse plug of 16-30 ampere rating. Of course, more than two variations may be provided, if desired. Figs. 4 and 6 represent fuse plugs of different ratings with the difference in the exterior diameter of their body parts 3D exaggerated to make the difference visually apparent. Figs. 5 and 7 illustrate insulating sleeve elements 34 having axial holes respectively of size to nicely receive the body 30 of different plugs.

Obviously the Fig. 7 sleeve element will reject the Fig. 4 fuse plug.

From the foregoing, it will be obvious that I have provided a fuse plug of the tamper-resisting variety which is operatively mountable in standard cut-out base receptacles without any Y present installations.

ladapter or ,other auxiliary Ainsert in .thereceptacle. Yet my .improved fusel plug `is readily com- .binable `with a tamper-resisting insulating sleeve -element, which may-be. inserted in the standard cut-out base receptacle, to provideall of the pro- Vfuse manufacturers to produce only .fuse .plugs V.of the tamper-resisting variety without any need Yfor continued manufacture'of prior conventional fuse plugs to meet the .replacement demands lof My improved fuse plug can effectively serve, without need for any adapter, all of the .replacement requirements of present installations, and can effectively meet the Code requirements Vfor new installations Vby the mere inclusion'ofan inexpensive tamper-resisting sleeve element which in no way affects the operative electrical .connections through a .fuse plug assembly.

`While I have shown a fuse plug having the mica disc 42 for visibility of the fusible element y vis well konwn in the art. Also, `I desire it to be understood that the axial hole 12 in the insulating sleeve elements 34, and the exterior surface s:

of the plug body part 3ll,may/be variously c0- operatively shaped so long as the center contact yof the fuse plug is adapted .to be projected entirely through the sleeve element into direct engagement with the center contact of `the base receptacle.

VIf desired, the end of .the body part 3!! of the plug and/or the entrance zend'of lthe sleeve elements 34 may be serrated or otherwise shaped to increase cutting or breaking coaction between vthe plug and sleeve as the plug enters the sleeve. Figs. 8 and 9 suggest such serrationsor teeth at '14 on the end of the body part 39 of a fuse plug, and Fig. 10 suggests similar serrations or teeth 1G on the outer end of a tamper-resisting sleeve element 34.

While an insert type of tamper-resisting sleeve is desirable for use in the standard cut-out base receptacle presently installed, it may be desirable in future cut-out base receptacles to'build the tamper-resisting sleeve in the base receptacle at the time of manufacture of the base receptacle. Fig. 11 illustrates a molded cut-out base receptacle structure whose main body 18 may be of Bakelite or any other suitable moldable insulating material, with the terminal elements 80, 82 embedded in the molded material. The screwshell contact 84 is electrically connected to terminal element 80, and the center Contact 85 is electrically connected to the terminal element 82. My preferred form of tamper-resisting sleeve 88, in this case, is molded as an integral part of the insulating body 18, with extent and arrangement within screw-shell contact `84 generally the same as in the previously described insert-type of Figs. 1, 5 and '7.

I claim as my invention:

1. A device for adapting a socket having a female screw-shell contact and a center contact and originally designed to receive a particular type of fuse plugs, for reception of only specially designed fuse plugs, said device comprising a generally tubular insulating element exteriorly threaded from end to end and having continuity of its threads from end to end, the threads at one .end portion .having -dia'meter for .screwing into said socket .screw-shell contact and the .threads at the other end portion having smaller .diameter for screwing .into a fuse plug screw-shell contact which itself .may .be screwed into said socket .screw-.shell contact. Y v

2. Ina tamper-resistingfuse .plug and socket assembly, a .fuse .plug screw-shell contact and .a

socket screw-.shell 4contact adapted .for .screwthread engagement, .one within the other, .and .an exteriorly4 threaded .insulatingmember having .its threads at .one region adapted to threadedly engage .the outer one oisaidscrew-.shell .contacts and having its threads-.at another. region adapted to threadedly .engage the innerone .ofsaid .screw- :shellcontacts with said innerscrew-shell contact intervening between `the outer screw-shellcontact and said insulating member at the lattersaid region.

3. In atamper-ressting .fuse plug .and socket assembly, a fuse plug -having a plug ,screw-shell contact and an `insulating body 4part .extending axially inspaced relation to said plug screw-shell contact, a plug center contact at the .projected end of said insulating body part, a socket .having a socket center contact and having .a socket screw-shell contact adapted to .threadedly lengage said plug .screw-shell contact Vthereby vto directly engage saidplug center contact with said socket center contact, andan .insulatingsleeve member disposed within said socket with one end portion surrounding ,said socket center contact and exteriorly threaded vfor .threadedly engaging said socket screw-shell contact, the outer .end

f portion of said sleeve member having smaller diameter and `being exteriorly threaded lfor threadedly engagingsaid plug screw-.shell .contact.

4. In a plugfuse and socketassembly, a fuse plug .having an insulating body .part carrying a plug center contactatits end and having a plug screw-shell contact in surrounding vspaced .relation to saidinsulating .body part whereby a substantial annular chamber is formed within the plug screw-shell contact, a socket having a socket center contact and having a socket screw-shell Contact within which said plug screw-shell contact may be screwed to directly engage the two said center contacts, an insulating sleeve having an inner end portion surrounding said socket center contact and having an outer end portion for entering said annular chamber of the plug, said inner end portion of the sleeve being exteriorly threaded for screwing into said socket screw-shell contact and said outer end portion of the sleeve being exteriorly threaded for threadedly engaging said plug screw-shell contact when the latter is screwed into said socket screw-shell contact.

5. In a fuse plug base receptacle having a screw-shell contact, a tamper-resisting means within said screw-shell contact comprising a body of insulating material having a substantial portion with outside diameter only slightly less than the interior diameter of said screw-shell contact and having its exterior threaded, whereby said substantial portion of the insulating body is adapted to threadedly engage the inside while said screw-shell contact of the receptacle threadedly engages the outside of the threads of a screw-shell contact of a fuse plug when such a plug is screwed into the base receptacle.

6. A fuse plug assembly comprising a base having center and screw-shell contacts, a tamperresisting insulating sleeve element having an exteriorly threaded inner end portion for screwing into said screw-shell contact of the base, and having a portion of less exterior diameter exteriorly threaded and projecting a substantial distance in axial direction outward from said threaded inner end portion to provide an annular space` between said screw-shell contact and r.said threaded outward projecting portion of the sleeve element, said sleeve element having an axial passage opening through its opposite ends and the walls of said passage, at the inner end of the sleeve element, being adapted to surround said center contact of the base when the sleeve element isscrewed into said screw-shell contact of the base, and a fuse plug having a screw-shell contact for screwing into said screw-shell contact of the base and for simultaneously screwing on said exteriorly threaded outward projecting portion of the sleeve element, said fuse plug having a hollow center-contact-carrying portion of insulating materialprojecting axially within and beyond the screw-shell contact of the plug, in spaced relation thereto, and said hollow centercontact-carrying portion of the plug being adapted to pass through the axial passage of the sleeve element, and there being a center contact exposed only at the projected end of the center-contactcarrying portion of the plug for directly engaging the center contact of the base when it is projected through said axial passage in the sleeve element.

7. In a fuse plug assembly having a fuse plug mounted in a base receptacle with a screw-shell contact of the plug screwed into a screw-shell contact of the base receptacle and with a center contact of the plug directly engaged with a center contact of the base receptacle, tamper-resist ing means in the base receptacle comprising an insulating sleeve element having an axial passage opening through its opposite ends and hav ing insulating walls of the passage surrounding the said engaged center contacts of the base receptacle and plug, said insulating sleeve element having one portion exteriorly threaded and screwed into the screw-shell contact of the plug, and having another portion exteriorly threaded and screwed into the screw-shell contact of the base receptacle.

8. An insulating tamper-resisting insert for a fuse plug base receptacle, comprising a sleeve element of insulating material having an axial passage opening through its opposite ends, exterior screw threads extending continuously from end to end of the sleeve element,'said screw threads at one end portion of the sleeve element having diameter for screwing into a screw-shell contact of the base receptacle and said screw threads at the other end portion of the sleeve element having smaller diameter for screwing into a screw-shell contact of a fuse plug when said screw-shell contact of the fuse plug is screwed into said screwshell contact of the base receptacle.

- ELMER H. TAYLOR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 656,454 Froschl et al Aug. 21, 1900 1,706,150 Floring Mar. 19, 1929 1,782,459 Brown Nov. 25, 1930 1,916,127 Newman June 27, 1933 1,990,953 Moore Feb. 12, 1935 2,115,447 McEntee Apr. 26, 1938 2,147,221 Taylor Feb. 14, 1939 2,159,083 Hagendorn et al. May 23, 1939 2,186,920 Currie Jan. 9, 1940 2,229,443 Dallam Jan. 21, 1941 2,419,453 Kocevar' Apr. 22, 1947 FOREIGN PATENTS Number Country Date 113,434 Germany Sept. 15, 1900 

